N-methyl-D-aspartate (NMDA) receptors are ligand-gated ion channels located primarily within the central nervous system (CNS). They belong to the family of ionotropic glutamate receptors and exist as multiple subtypes due to the different combinations of subunits—NR1, NR2 (NR2A, NR2B, NR2C, NR2D) and NR3—that can be expressed. In addition to the agonist binding site, NMDA receptors have multiple distinct binding sites for various compounds that enhance, modulate and inhibit the activation of the receptors.
It is known that NMDA receptors are involved in neuronal communication and play important roles in synaptic plasticity and mechanisms that underlie learning and memory. Under normal conditions, NMDA receptors engage in synaptic transmission via the neurotransmitter glutamate, which regulates and refines synaptic growth and plasticity. However, when there are abnormally high levels of glutamate (i.e. under pathological conditions), NMDA receptors become over-activated, resulting in an excess of Ca2+ influx into neuronal cells, which in turn causes excitotoxicity and the activation of several signaling pathways that trigger neuronal apoptosis. Glutamate-induced apoptosis in brain tissue also accompanies oxidative stress resulting in loss of ATP, loss of mitochondrial membrane potential, and the release of reactive oxygen species and reactive nitrogen species (e.g. H2O2, NO, OONO−, O2−) causing associated cell damage and death. Decreased nerve cell function and neuronal cell death eventually occur. Excitotoxicity also occurs if the cell's energy metabolism is compromised.
Over-activation of the NMDA receptors is implicated in neurodegenerative diseases and other neuro-related conditions as it causes neuronal loss and cognitive impairment, and also plays a part in the final common pathway leading to neuronal injury in a variety of neurodegenerative disorders such as amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease and Huntington's disease, as well as conditions such as stroke. Recent findings have implicated NMDA receptors in many other neurological disorders, such as multiple sclerosis, cerebral palsy (periventricular leukomalacia), and spinal cord injury, as well as in chronic and severe mood disorders (Mathew S J et al., Rev Bras Psiquiatr, 27:243-248 (2005)).
NMDA receptors have played crucial roles in both regulating and promoting normal nervous system functions as well as in causing cell-death, which leads to lethal conditions. There has been increasing evidence that the type of signal given to a cell depends on the location of the activated NMDA receptor. Growth and survival-promoting signals result from the activated synaptic NMDA receptors, while cell death causing signals result from the extrasynaptic NMDA receptors. Recent studies also indicate that the activated synaptic NMDA receptors lead to robust phosphorylation of the transcription factor CREB on the transcriptional regulatory residue Ser133 and promote CREB-dependent gene expression and neuronal survival. However, the activated extrasynaptic NMDA receptors transiently phosphorylate CREB and do not activate CREB-dependent gene expression, resulting in neuronal cell death (Hardingham G E et al., Nat Neurosci, 5: 405-414 (2002)).
Yet, there are few effective therapeutic agents for excitotoxicity to alleviate symptoms of its associated neuronal disorders. One complication for the development of effective NMDA antagonists as neurotherapeutic drugs is that many NMDA antagonists also exhibit psychotogenic and neurotoxic properties. For example, MK-801 (dizocilpine maleate) is capable of providing certain degree of neuroprotection in ischemic stroke, but is associated with psychotropic and adverse motor effects. Thus, it is desirable to identify and/or to develop compounds that can potentiate NMDA synaptic activity resulting in neuroprotection.
Melanocortins (MC) receptors are a class of G protein coupled receptors. MC are a group of pituitary peptide hormones, which include adrenocorticotropic hormone (ACTH) and the alpha, beta and gamma melanocyte-stimulating hormones (MSH). They are derived from the pro-hormone proopiomelanocortin (Adan et al., (2000) Melanocortins and the brain: from effects via receptors to drug targets. Eur J Pharmacol 405: 13-24). MCs act through a multitude of melanocortin receptors designated MC1 through MC5. MC1 receptors are expressed in macrophages and monocytes, keratinocytes and melanocytes, endothelial cells, glioma cells and astrocytes, and pituitary and periaqueductal grey matter, where they are involved in melanogenesis and anti-inflammatory processes (Kang et al., (2006) A selective small molecule agonist of the melanocortin-1 receptor inhibits lipopolysaccharide-induced cytokine accumulation and leukocyte infiltration in mice. J Leukoc Biol 80: 897-904; and Slominski et al., (2004) Melanin pigmentation in mammalian skin and its hormonal regulation. Physiol Rev 84: 1155-228). They are also found in subcutaneous fat of obese subjects and thought to play a role in the pathophysiology of obesity (Hoch et al., Expression and localization of melanocortin-1 receptor in human adipose tissues of severely obese patients. Obesity (Silver Spring) 15: 40-9). ACTH binds to MC2 receptor (ACTH receptor) and mainly expressed in adrenal gland and adrenal cortex. While MC3 is expressed in both periphery and neural tissues, MC4 is mainly found in CNS and is the second neural MC receptor as they are expressed in multiple regions of the brain including the cortex, thalamus, hypothalamus, brainstem, and spinal cord. The receptor is also highly expressed in the paraventricular nucleus and is involved in the modulation of pituitary function. MC5, highly homologous to MC4, is the only MC receptor found in skeletal muscle and is broadly expressed in periphery and present in specific brain regions.
MC4 receptor activity has been linked to neurite outgrowth and peripheral nerve regeneration (Tanabe et al., (2007) Melanocortin receptor 4 is induced in nerve-injured motor and sensory neurons of mouse. J Neurochem 101:1145-52; and Adan et al., (1996) Melanocortin receptors mediate alpha-MSH-induced stimulation of neurite outgrowth in neuro 2A cells. Brain Res Mol Brain Res 36:37-44), cognitive functions and neuroprotection in brain ischemia stroke (Giuliani et al., 2006), inflammatory responses in astrocytes (Caruso et al., (2007) Activation of melanocortin 4 receptors reduces the inflammatory response and prevents apoptosis induced by lipopolysaccharide and interferon-gamma in astrocytes. Endocrinology 148: 4918-26). Studies conducted on the heptapeptide Semax (Met-Glu-His-Phe-Pro-Gly-Pro; SEQ ID NO:1)—an analog of the adrenocorticotropin fragment (4-10) but lack ACTH hormonal activity, is an antagonist for MC4 receptor (Adan et al., (1994) Identification of antagonists for melanocortin MC3, MC4 and MC5 receptors. Eur J Pharmacol 269: 331-7). Semax has been reported to enhance cognitive brain functions by modulating the expression and the activation of the hippocampal BDNF/trkB system (Tsai, (2007) Semax, an analogue of adrenocorticotropin (4-10), is a potential agent for the treatment of attention-deficit hyperactivity disorder and Rett syndrome. Med Hypotheses 68: 1144-1146; and Dolotov et al., (2006) Semax, an analogue of adrenocorticotropin (4-10), binds specifically and increases levels of brain-derived neurotrophic factor protein in rat basal forebrain. J Neurochem 97 Suppl 1: 82-86). BDNF has long been known for its involvement in learning and memory, modulation of dendritic spine density and morphology, regulation of axonal growth, and various therapeutic strategies for neurological disorders have targeted BDNF (Winckler, (2007) BDNF instructs the kinase LKB1 to grow an axon. Cell 129: 459-60; Ji et al., (2005) Cyclic AMP controls BDNF-induced TrkB phosphorylation and dendritic spine formation in mature hippocampal neurons. Nat Neurosci 8: 164-72; Pezet et al., (2004) Brain-derived neurotrophic factor as a drug target for CNS disorders. Expert Opin Ther Targets 8: 391-399; Yamada et al., (2003) Brain-derived neurotrophic factor/TrkB signaling in memory processes. J Pharmacol Sci 91: 267-270). BDNF expression is also linked to stress and depression, and various treatments for depression (such as antidepressants and electroconvulsive therapy) work by inducing BDNF expression in the brain (reviewed in Castren et al., (2007) Role of neurotrophic factors in depression. Curr Opin Pharmacol 7: 18-21; Kuipers et al, (2006) Brain-derived neurotrophic factor mechanisms and function in adult synaptic plasticity: new insights and implications for therapy. Curr Opin Drug Discov Devel 9: 580-586; and Malberg et al., (2005) Antidepressant action: to the nucleus and beyond. Trends Pharmacol Sci 26: 631-638). As such, MC4 receptor antagonism has been postulated as a therapeutic mechanism against depression, anxiety and cachexia (Chaki et al., (2007) Melanocortin-4 receptor antagonists for the treatment of depression and anxiety disorders. Curr Top Med Chem 7: 1145-1151; and Foster et al., (2007) Melanocortin-4 receptor antagonists as potential therapeutics in the treatment of cachexia. Curr Top Med Chem 7: 1131-1136).
Therefore, there is a need to develop effective NMDA and MC4 antagonists that have high potency and are capable of (i) preventing and/or treating the CNS disorders, such as excitotoxicity, neurodegenerative diseases and neuropathological conditions; (ii) providing neuroprotection under stress conditions, such as a stroke; (iii) enhancing the brain's cognitive functions; and (iv) offering treatment to conditions, such as depression, anxiety, anorexia and cachexia induced by other chronic diseases. The present invention satisfies this and other needs.